The preparation of aldehydes and alcohols by adding carbon monoxide and hydrogen to olefinic double bonds (hydroformylation) is known. The reaction is catalyzed by metals of Group VIII of the Periodic Table (IUPAC version) or compounds thereof, and forms carbonyls or hydridocarbonyls under the reaction conditions. Formerly, cobalt and cobalt compounds were employed almost exclusively as the catalysts; nowadays rhodium catalysts are used increasingly, even though rhodium is several times more expensive than cobalt. Rhodium is used in this reaction on its own or in combination with complexing agents, for example organic phosphines. Whereas the oxo process using rhodium as catalyst requires reaction pressures of 25 to 30 MPa, pressures of 1 to 5 MPa are sufficient if rhodium complex compounds are employed.
In many cases rhodium catalysts give rise to distinct advantages. They possess a higher activity and selectivity. Furthermore, they enable the manufacturing plant to be operated in many respects free from problems, in particular as to the operation of the synthesis and the discharge of the products from the reactor. Finally, the conventional oxo process based on cobalt catalysts can be switched over in many cases, using the existing pieces of equipment, to rhodium catalysts with only minor capital expenditure.
Considerable difficulties are, however, experienced in achieving the loss-free--or at least approximately loss-free--separation and recovery of the rhodium, irrespective of whether it is employed with or without additional complexing agents. After the conclusion of the reaction, the rhodium is dissolved in the hydroformylation product in the form of the carbonyl compound. It can, in certain cases, also contain further ligands.
The crude oxo product is usually worked up by reducing the pressure in several stages, by initially reducing the synthesis pressure (which is about 1 to 30 MPa, depending on the type of rhodium catalyst employed) to about 0.5 to 2.5 MPa. In the course of this, synthesis gas dissolved in the crude product is set free. It is then possible to reduce the pressure to normal. The removal of the rhodium is carried out either directly from the crude product or from the residue of the distillation of the crude product.
The first route is followed if rhodium without additional complexing agents has been employed as the catalyst in the preceding hydroformylation stage. The second variant is used if the rhodium catalyst also contains, in addition to carbon monoxide, further ligands, such as phosphines or phosphites, in a complex combination. It can also be used if, although the hydroformylation has been carried out using rhodium alone, a complexing agent has been added to the crude product after releasing the pressure in order to stabilize the rhodium. Basically, it must be borne in mind that the noble metal is present in the crude product in a concentration of only a few ppm, and its removal therefore requires a very meticulous operation. In addition, difficulties can arise from the fact that the rhodium sometimes changes into the metallic form, or forms multinuclear carbonyls when the pressure is released, particularly if it has been employed without a ligand. A heterogeneous system composed of the liquid organic phase and the solid phase containing rhodium or rhodium compounds is then formed.
The recovery of rhodium from the products of the oxo process, including the residues of the crude oxo products, has been investigated many times. Research has led to the development of numerous processes, of which a few have also been used on an industrial scale.
The subject of U.S. Pat. No. 4,400,547 is the hydroformylation of olefins having 2 to 20 carbon atoms in the presence of unmodified rhodium as catalyst. After the conclusion of the reaction, a complex-forming compound such as triphenylphosphine is added to the crude oxo product and the aldehyde is distilled off. The distillation residue is then treated with oxygen in order to split the ligands back out of the complex and recover the rhodium in an active form. Separation of the rhodium and the distillation residue is not possible by this procedure.
The removal of noble metals such as rhodium from high-boiling hydroformylation residues is also described in U.S. Pat. No. 3,547,964. For this purpose, the residues are treated with hydrogen peroxide in the presence of acids, such as formic acid, nitric acid, or sulfuric acid. Limits are, however, set to the industrial use of the process on account of the high cost of hydrogen peroxide and the problems associated with its handling.
In accordance with DE 24 48 005 C2, a distillation residue containing rhodium is first treated with acids and peroxides. Excess peroxides are then destroyed by heating, and the aqueous solution containing the catalyst metal is reacted, in the presence of a water-soluble organic solvent, with hydrogen halide acid or alkali metal halides and also with tertiary phosphines and carbon monoxide or compounds which donate carbon monoxide. Once again, this procedure requires the use of peroxides, with the disadvantages described above, and the use of materials of construction stable to halogens.
Finally, EP 15,379 B1 describes a process for regenerating a rhodium catalyst which contains ligands and which has been deactivated in a hydroformylation reaction. This is effected by adding an aldehyde to the catalyst in an amount such that at least 1 mol aldehyde is present per mole of rhodium and per mole of ligand. Oxygen in the form of air is then passed through the mixture of catalyst and aldehyde, the solid oxidized ligand is removed, and the ratio of ligand to rhodium is adjusted to the value desired for the hydroformylation reaction. Although this procedure permits the activity of the catalyst to be restored, it does not make possible either the recovery of the rhodium or the elimination of the impurities which are not oxidized by the action of oxygen or air or which do not give conversion products sparingly soluble or insoluble in the organic medium.